1. Field of Invention
The present invention relates to a fixing apparatus used in an image forming apparatus such as a copier, a printer, and a facsimile apparatus; and particularly relates to a fixing apparatus having a fixing heater that heats a fixing roller from inside the fixing roller, and to an image forming apparatus provided with such a fixing apparatus.
2. Related Art
A conventional image forming apparatus such as a copier, a printer, and a facsimile apparatus is configured, for example, so as to form a toner image on a sheet such as recording paper using a toner, and to heat and fix the obtained unfixed toner image on the sheet using a fixing apparatus. For example, in a conventional image forming apparatus in which image-forming is performed by an electrophotographic image-forming process, an image is formed on a sheet by charging the surface of an image carrier such as a photosensitive body, forming an electrostatic latent image in the charging area by exposing it to an image, electrostatically transferring a toner image that is formed by developing the electrostatic latent image using a developer to the sheet, and fixing the unfixed toner image that was transferred to the sheet using the fixing apparatus.
In a fixing apparatus used in such a conventional image forming apparatus, generally, an unfixed toner image on the sheet is fixed by allowing the unfixed toner image on the sheet to pass through a fixing nip portion between a hot roller and a pressure roller of the fixing apparatus.
To be specific, conventional fixing apparatuses are generally provided with a fixing heater inside the fixing roller, and fixing is performed by heating the fixing roller using such an internal fixing heater to a predetermined fixing temperature, and pressing and heating the sheet with the unfixed toner image formed thereon while allowing the sheet with the unfixed toner image to pass through the fixing nip portion between the heated fixing roller and pressure roller, so as to bring the unfixed toner image into contact with the fixing roller. However, when performing this fixing operation, sometimes the temperature distribution declines in a part of the region of the fixing roller in the longitudinal direction to a temperature lower than the fixing temperature in that part.
In view of the above, in a conventional fixing apparatus, a plurality of fixing heaters having heat distribution characteristics that are different from each other are provided inside the fixing roller, and the temperature distribution of the fixing roller in the longitudinal direction is kept at the fixing temperature.
For example, JP2003-280444A has proposed a fixing apparatus that prevents temperature drops that may occur due to poor temperature responsiveness in a partial region of a fixing roller in the longitudinal direction by differentiating the heat distribution of a plurality of heating means (fixing heaters) and performing control so as to shift the state of a heating means from a non-operating state to an operating state in order from a heating means with a concentrated heat distribution amount at the center portion of the paper passage area among the plurality of heating means, not simultaneously shifting the state of the heating means from a non-operating state to an operating state, or not simultaneously shifting the state of the heating means from an operating state to a non-operating state.
Meanwhile, power distribution to the fixing heater is stopped while the main power supply is OFF and the fixing roller has a normal temperature (a temperature not heated by the fixing heater). Furthermore, while in a stand-by (power-saving: energy-saving) mode in which an image-forming operation is not performed, the stand-by temperature of the fixing heater is controlled, and the temperature of the fixing roller is kept at a stand-by temperature (a predetermined temperature that is lower than the fixing temperature).
Then, when the main power supply that has been turned OFF is turned ON, during the warm-up period until the fixing roller in a lower temperature (normal temperature) state reaches the fixing temperature (until entering ready state), power is distributed to the fixing heater; and upon entering the ready state, fixing temperature control is performed on the fixing heater so as to keep the temperature of the fixing roller at the fixing temperature. Meanwhile, when a stand-by mode cancel instruction is given, control of the fixing heater is switched from the stand-by temperature control to the fixing temperature control, during the warm-up period which is until the fixing roller in a lower temperature state (having a stand-by temperature) reaches the fixing temperature. The temperature of the fixing roller in the low temperature state is thus raised so that the fixing heater has a fixing temperature.
During this warm-up period, it is required that the fixing roller is given an appropriate temperature that can ensure fixing quality (excellent fixing ability) in a short period of time. Therefore, recently, the electrical power (calorific value) of the heater has increased significantly.
However, although the time required to raise the temperature to a required temperature condition can be shortened by quickly heating the fixing roller using the fixing apparatus having a fixing heater with increased electrical power (calorific value), there is a limitation in increasing the capacity of the fixing heater because consideration needs to also be given to, for example, limitations from the current ratings and the like of indoor circuit breakers provided at the place where the apparatus is located.
Thus, an important task is to efficiently raise the temperature of the fixing roller during the warm-up period somehow. The same goes for the above-described fixing apparatus, that is, the fixing apparatus having a plurality of fixing heaters having heat distribution characteristics that are different from each other inside the fixing roller.
From such a standpoint, although the fixing apparatus described in JP2003-280444A can prevent a temperature drop that may occur due to poor temperature responsiveness in a partial region of the fixing roller in the longitudinal direction, no consideration is given to efficiently raising the temperature of the fixing roller during the warm-up period.